1. Field of the Invention
The present invention relates to a technique of damping a peak level of a transfer function of a low-pass filter provided on the DC input side of a power converter, and, in particular, to a resonance damping apparatus for efficiently damping a resonance Q of an LC filter inserted between the DC input side of a DC/AC inverter (or DC/DC converter) for electric railcars and a power line.
2. Description of the Related Art
An electric railcar employs a power conversion apparatus, for example, an inverter apparatus, as a power source for supplying power to loads such as electric motors for driving the railcar, various control devices, air conditioners and illumination devices. The power conversion apparatus converts a high line voltage supplied through a pantagraph to a low voltage suitable for each load.
The inverter apparatus of this type comprises an LC type DC filter having a low-pass characteristic in order to eliminate a high-frequency noise component contained in a line voltage supplied through a pantograph. The filter has a positive gain characteristic having a peak as high as about +20dB in a range of about 50 to 220 rad/sec (8-35 Hz).
The above-described power conversion apparatus having the inverter apparatus is well known, for example, as disclosed in Published Unexamined Japanese Patent Application (PUJPA) No. 63-174501.
When the load and/or DC input of the inverter apparatus abruptly changes, for example, when the load of the inverter or line voltage abruptly changes due to the starting of air-conditioners in the railcar, the ratio of voltages shared by the L (reactor) and C (capacitor) constituting the filter is changed. This change varies the DC input of the inverter apparatus, and consequently a fluctuation appears in the envelope of the inverter output voltage.
If the envelope of the inverter output voltage fluctuates, the load current of the inverter apparatus correspondingly varies and the DC input voltage of the inverter apparatus changes. Such a chain of a loop of the voltage/current changes causes a resonance phenomenon in the output voltage of the LC filter (DC input voltage of the inverter apparatus).
As described above, in the conventional power conversion apparatus for railcars, vehicles, etc., electric power is supplied to the inverter apparatus through the LC filter for eliminating a high-frequency component of the line voltage. Thus, if the periodic fluctuation in the input voltage due to a sudden load variation or a sudden line voltage change cannot completely corrected by a phase control circuit in the inverter apparatus, the output voltage oscillates periodically. As a result, a stable AC voltage cannot be supplied to an illumination device such as a fluorescent lamp, so that the illumination device may flicker.
The above-described resonance phenomenon can be prevented by the following methods:
(1) To increase the power capacity of the inverter apparatus thereby to reduce the influence due to the load variation;
(2) To increase the capacitance of the capacitor which constitutes the LC filter; or
(3) To connect a limit resistor having a relatively high resistance value in series to the LC filter.
In order to perform method (1) or (2), the volume of the apparatus must be increased. The apparatus with such a large volume is not suitable for use in the electric railcar. In the case of method (3), since the amount of heat radiated from the limit resistor is large, the efficiency of the inverter apparatus lowers. Thus, bare method (3) is unsuitable.